perl -n seemingly eating a line of input

mldvx4 has asked for the wisdom of the Perl Monks concerning the following question:

I would expect that the following would print out all five input lines:

echo -e '55\n44\n33\n22\n11\n' | perl -n -e '@a = <>; END{ print @a; } '

Instead, the first line gets eaten. The first line always gets eaten regardless of how many numbers I send to perl.

What is the explanation for the behavior? What should be changed so that all numbers sent to perl end up in @a?

Comment on perl -n seemingly eating a line of input Download Code

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Re: perl -n seemingly eating a line of input by shmem (Chancellor) on May 01, 2017 at 11:42 UTC
This happens because at the time your loop code `@a = <>;` is run, there has already been an assignment to `$_`. `perl -MO=Deparse -n -e '@a = <>; END{ print @a; } ' LINE: while (defined($_ = <ARGV>)) { @a = <ARGV>; sub END { print @a; } ; }` [download] This captures the first line also into `@a`: `echo -e '55\n44\n33\n22\n11\n' \| perl -n -e '@a = ($_,<>); END{ print +@a; } '` [download] perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'	[reply] [d/l] [select]
Re^2: perl -n seemingly eating a line of input by mldvx4 (Friar) on May 01, 2017 at 12:29 UTC
Thanks. I see the problem now, though don't see the way around it. Adding the parenthesis catches the first number, but also produces an extra line that is empty.	[reply]
Re^3: perl -n seemingly eating a line of input by shmem (Chancellor) on May 01, 2017 at 13:18 UTC
...but also produces an extra line that is empty. That is due to your trailing `\n` and has nothing to do with the parenthesized `$_,<>` - compare `echo -e '55\n44\n33\n22\n11\n' \| perl -n -e '@a = ($_,<>); END{ print +@a; } ' echo -e '55\n44\n33\n22\n11' \| perl -n -e '@a = ($_,<>); END{ print @a +; } '` [download] - because `echo -e` will add a newline to its argument, so there are two of them, and thus an empty line. Use `echo -en` to suppress that: `echo -en '55\n44\n33\n22\n11\n' \| perl -n -e '@a = ($_,<>); END{ print + @a; } '` [download] perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'	[reply] [d/l] [select]
Re^3: perl -n seemingly eating a line of input by bart (Canon) on May 01, 2017 at 13:00 UTC
Don't use `-n`, or change the way you process the lines: `echo -e '55\n44\n33\n22\n11\n' \| perl -n -e 'push @a, $_; END{ print @ +a; } '` [download] Explanation: `-n` makes perl loop through the input lines, and my code puts each line in turn onto the array. After all input is processed, I print out the array.	[reply] [d/l] [select]
Re^3: perl -n seemingly eating a line of input by Anonymous Monk on May 01, 2017 at 12:46 UTC
If you want to do `-e '@a=<>;'` and get all the lines, simply don't use `-n`.	[reply] [d/l] [select]
Re: perl -n seemingly eating a line of input by ikegami (Patriarch) on May 01, 2017 at 15:47 UTC
perl -n seemingly eating a line of input Well yeah, that's the point of `-n`. Specifically, `-n` wraps your code with `LINE: while (<>) { ... }` [download] Fix (with `-n`): `perl -ne'push @a, $_; END { print @a; }'` [download] Fix (without `-n`): `perl -e'@a = <>; print @a;'` [download]	[reply] [d/l] [select]
Re: perl -n seemingly eating a line of input by Anonymous Monk on May 01, 2017 at 22:44 UTC
TIMTOWTDI! (just move trailing newline to beginning of string) `xD` `echo -e '\n55\n44\n33\n22\n11' \| perl -ne'@a = <>; END{ print @a; }'` [download]	[reply] [d/l]

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